1. Field of the Invention
The present invention relates to an internal-combustion-engine combustion condition detection apparatus, and more particularly to an internal-combustion-engine combustion condition detection apparatus that can securely determine whether or not an ignition-plug smolder and/or preignition have occurred.
2. Description of the Related Art
In operating an internal combustion engine, in the case where a carbon deposit, which occurs when a mixed gas (fuel-air mixture) in a cylinder imperfectly combusts, adheres to the surface of the insulator for an ignition-plug ignition portion, the value of the insulation resistance across the electrodes of the ignition plug decreases, whereby a spark becomes unlikely to occur.
This phenomenon is commonly known as “soiling of an ignition plug due to smoldering”.
In addition, the phenomenon is referred to as “smoldering” in which the value of the insulation resistance across the electrodes of an ignition plug decreases and thereby a leakage current occurs across the electrodes of the ignition plug.
Additionally, due to combustion in a combustion chamber, the molecules of a mixture gas in the combustion chamber ionize, and when a voltage is applied to the ionized combustion chamber through the ignition plug, a minute current flows. The minute current is referred to as an ion current.
To date, it has been known that, in an spark-ignition internal combustion engine, an ion current that occurs in a combustion chamber after the start of ignition through an ignition plug is detected, the driving condition, such as a knock or a combustion limit, of the internal combustion engine is detected through the magnitude of the detected ion current, the time period during which the ion current occurs, or the like, and based on the result of the detection, the ignition timing is adjusted or the amount of a fuel to be injected is corrected.
In such an ion current detection method utilizing an ignition plug, an ion current can be detected each time ignition is executed, as long as no abnormality exists in the ignition plug.
However, as the soiling of an ignition plug due to smoldering advances, the insulation resistance value of the ignition plug remarkably decreases, whereby a leakage current across the electrodes of the ignition plug increases.
Accordingly, a case may occur in which, even when, due to a misfire, no ion current occurs, a leakage current is detected as an ion current, whereby the misfire cannot be detected.
In addition, it is commonly known that soiling due to a carbon deposit has a self-cleaning action in that the soiling occurs when the temperature of an ignition plug is low and the engine is in a state in which the rotation speed is low and the engine load is small, and when the temperature of the ignition plug rises, the carbon deposit that has adhered to the surface of the insulator for the ignition portion of the ignition plug is burned off.
Accordingly, it is an effective method to facilitate increase in the temperature of an ignition plug, in terms of improving smoldering due to carbon-deposit soiling.
Additionally, there exists a phenomenon in which, in driving an internal combustion engine, a hot spot caused by a residual temperature of a carbon deposit that has adhered to an ignition plug or to the inside of a cylinder makes a mixture gas spontaneously catch fire halfway through a compression stroke.
The foregoing phenomenon is referred to as preignition; preignition not only causes a sharp decrease in the output of an internal combustion engine or an imperfect rotation but also damages the internal combustion engine in the worst case.
FIG. 7 is a set of charts for explaining a problem in a conventional preignition detection method, for example, disclosed in Japanese Patent Publication No. 3176291 (Patent Document 1); FIG. 7 represents the relationship between the ion current and the leakage current when preignition occurs.
FIG. 7(a) represents a case in which a mixture gas normally catches fire through a discharge from an ignition plug. Firstly, a pulse occurs at each of the rising and the falling timing of an ignition signal; after that noise is caused by a discharge from the ignition plug; then, an ion current (combustion ion current) occurs.
FIG. 7(b) represents a case in which preignition occurs and then an ion current flows; the width of the pulse that occurs at the falling timing of the ignition signal is widened.
FIG. 7(c) represents a case in which a smolder occurs in the ignition plug; a leakage current flows in a secondary circuit not only as the ignition signal rises but also even after a discharge from the ignition plug.
FIG. 7(d) represents a case in which a smolder occurs in the ignition plug and preignition occurs; the respective pulses that occur at the rising and the falling timing of the ignition signal join with each other, whereby a pulse caused by preignition cannot be discriminated.
FIG. 8 is a chart for explaining the conventional preignition detection method disclosed in Patent Document 1.
The respective voltages that occur across a detection resistor at a time instant when a first predetermined time period ts (ts: smolder determination duration) elapses after a pulse-shaped ignition signal has been outputted from an ignition device and at a time instant when a second predetermined time period tp (tp: determination duration for determining whether or not a combustion ion current occurs due to preignition or the like), which is longer than the first predetermined time period, elapses after the pulse-shaped ignition signal has been outputted from the ignition device are read into a microcomputer, as a smolder-detection-timing voltage V(ts) and a preignition-detection-timing voltage V(tp).
In the case where the smolder-detection-timing voltage V(ts) is higher than a predetermined threshold voltage, a smolder has occurred in an ignition plug; therefore, because the smolder may cause an erroneous determination, the determination whether or not preignition has occurred is canceled.
In contrast, in the case where the smolder-detection-timing voltage V(ts) is the same as or lower than the predetermined threshold voltage, no smolder has occurred in the ignition plug; therefore, because no erroneous determination is performed, determination whether or not preignition has occurred is performed based on the preignition-detection-timing voltage V(tp).
In addition, as represented in FIG. 8, a leakage current starts to occur from an ignition energization start timing; the higher the level of the smolder is, the longer the duration of the leakage current becomes.
Additionally, the higher the level of preignition is, the longer the duration of an ion current caused by preignition becomes in a direction in which the time instant advances.
FIG. 9 is a diagram for explaining the configuration and the operation of a conventional ion-current detection device.
In FIG. 9, reference numeral 100 denotes an ignition plug; reference numeral 100a denotes an ion current that occurs in a combustion chamber; reference numeral 100b denotes a resistor (a smolder resistor) that is formed of a carbon deposit that occurs across the electrodes of the ignition plug 100 when a mixture gas imperfectly combusts. A leakage current flows through the smolder resistor 100b. 
Reference numerals 201, 20, 20a, 20b, 30, and 41 denote an ignition device, an ignition coil, a primary coil of the ignition coil 20, a secondary coil, a transistor, and an ion-current detection device, respectively.
In the ion-current detection device 41, reference numerals 42, 43, 44, and 45 denote a capacitor, a diode, a zener diode, and an ion current shaping circuit, respectively.
The ignition plug 100 is provided in the combustion chamber and connected to the negative-polarity end of the secondary coil 20b of the ignition coil 20. The positive-polarity end of the primary coil 20a is connected to a power source, and the negative-polarity end thereof is connected to the collector of the transistor 30 for current switching.
The emitter of the transistor 30 is connected to the ground, and the base thereof is connected to an ECU (control device) 301 that controls combustion.
The ion-current detection device 41 is configured with the capacitor 42 connected to the positive-polarity end of the secondary coil 20b, the diode 43 connected between the lower-potential end of the capacitor 42 and the ground, the zener diode 44 that determines a voltage that is charged across the capacitor 42, and the ion current shaping circuit 45.
In addition, the ion-current detection device 41, configured with the capacitor 42, the diode 43, and the zener diode 44, detects an ion current, based on electric charges accumulated across the capacitor 42.
Additionally, the ion current shaping circuit 45 converts an ion current detected by the ion-current detection device 41 into a voltage and filters out noise components of a voltage-converted signal so as to shape the waveform thereof.
FIG. 10 is a set of charts representing the worst case of the relationship between the leakage current due to a smolder and the ion current when preignition is detected.
FIG. 10(a) represents an ignition signal, and FIG. 10(b) represents a secondary voltage that occurs across the secondary coil 20b of the ignition coil 20.
The ignition signal is applied to the base of the transistor 30 illustrated in FIG. 9; at the time instant when a current starts to flow through the primary coil 20a, an induction voltage of several kilovolts (e.g., approximately 1 kV) occurs across the secondary coil 20b; after that, the value (in this case, 140V) of the voltage across the zener diode 44 is determined by the voltage charged across the capacitor 42.
FIG. 10(c) represents a leakage current caused by a low-level smolder; unlike the state represented in FIG. 7(c), in the case where a low-level smolder occurs, a leakage current disappears halfway in the duration of the ignition signal.
Accordingly, in the case where a low-level smolder occurs, a leakage current can be detected only in the first half of the duration of the ignition signal, which is a short duration.
FIG. 10(d) represents an ion current when preignition occurs; FIG. 10(d) represents a case in which more runaway preignition occurs than in FIG. 7(b).
FIG. 10(e) represents the compression stroke range and the expansion stroke range of an internal combustion engine.
FIG. 11 is a diagram conceptually illustrating the configuration of an internal-combustion-engine combustion condition detection apparatus utilizing a conventional ion-current detection device.
In FIG. 11, reference numeral 100 denotes an ignition plug; reference numeral 201 denotes an ignition device that ignites by use of the ignition plug 100 a fuel-air mixture taken in for performing combustion when the internal combustion engine is operated.
Reference numeral 311 denotes an ignition control device that generates a control signal for controlling the operation of the ignition device 201.
Reference numeral 303 denotes an A/D converter that converts an ion current detected by the ion-current detection device 41 illustrated in FIG. 9 or a leakage current into a digital signal.
Reference numeral 314 denotes a leakage current detection range setting device that sets an ignition-plug smolder detection range; reference numeral 315 denotes a leakage current determination device that determines whether or not an ignition-plug smolder exists, based on a current detected within a detection range set by the leakage current detection range setting device 314; reference numeral 316 denotes an ion current detection range setting device that sets an ion-current detection range; reference numeral 317 denotes a preignition detection device that detects preignition or a precursor phenomenon of preignition, based on an ion current within a detection range set by the ion current detection range setting device 316.
In addition, reference numeral 301 denotes an ECU that is a control device.
FIG. 12 is a chart for explaining the timings for a smolder determination and a preignition determination in the foregoing conventional internal-combustion-engine combustion condition detection apparatus.
As represented in FIG. 12 or FIG. 8, to date, a smolder determination has been performed in the first half of the duration of an ignition signal, and a preignition determination has been performed in the second half of the duration of the ignition signal.
In other words, the leakage current detection range setting device 314 sets a leakage-current detection range in the first half of the duration of an ignition signal, and the ion current detection range setting device 316 sets a preignition detection range in the second half of the duration of the ignition signal.
In addition, in FIG. 12, “A” indicates a leakage-current detection range for a smolder determination, and “B” indicates an ion-current detection range for a preignition determination.
In a conventional internal-combustion-engine combustion condition detection apparatus, a smolder determination (i.e., a determination whether or not a leakage current exists) is performed in the first half of the duration of an ignition signal, and a preignition determination is performed in the second half of the duration of the ignition signal.
However, a leak current starts to occur from an ignition energization start timing; the higher the level of a smolder is, the longer the duration of the leak current becomes.
Additionally, the higher the level of preignition is, the longer the duration of an ion current caused by preignition becomes in a direction in which the time instant advances.
Therefore, the duration of a leakage current caused by a smolder and the duration of a combustion ion current caused by preignition or the like may overlap each other; in this case, neither a smolder detection nor a preignition detection can securely be performed.
Moreover, because the leakage-current detection range for a smolder determination and the ion-current detection range for a preignition determination cannot be set wide, it is difficult to raise the determination accuracy.